In general, natural rubber is frequently used in many fields including industrial products such as tires, rubber belts, rubber rolls, bladders and fenders and sporting goods such as tennis balls, basket balls, soccer balls and volley balls. In a tire, it is used as a material for various components constituting rubber tires, such as treads, side walls, ply coating rubbers and bead fillers.
In recent years, enzyme treatment techniques for natural rubber have been proposed for the purpose of removing protein contained in natural rubber. A deproteinizing enzyme treatment technique with protease has so far been used as an enzyme treatment technique for natural rubber latex (refer to Japanese Patent Application Laid-Open No. 56902/1994 and Japanese Patent Application Laid-Open No. 56906/1994).
Protein is removed from natural rubber latex for the purpose of not only removing a substance which is a cause to bring about an allergic symptom but also improving the natural rubber.
Natural rubber is more excellent in mechanical characteristics than synthetic rubber and has small so-called tan δ (dynamic loss factor). Accordingly, natural rubber is excellent in a low hysteresis loss property, but it is desired to be improved in a gripping property in a wet state. Further, crude natural rubber (rubber used as a raw material) is inferior in processability and a productivity in the production. In order to solve such problems, a deproteinizing technique in which a total nitrogen content of a natural rubber is reduced to 0.1 mass % or less has been proposed (refer to Japanese Patent Application Laid-Open No. 329838/1994). In conventional deproteinizing treatments, natural rubber latex is repeatedly washed with surfactants, and washing thereof is accompanied with centrifugal process.
For example, anionic surfactants and/or nonionic surfactants can be used as a surfactant used for deproteinizing treatment. The anionic surfactants include, for example, surfactants of a carboxylic acid base, a sulfonic acid base, a sulfate base and a phosphate base.
On the other hand, non-rubber components of about 6 mass % such as a protein component are contained in natural rubber. Lipid, sugar, fibrous matters and inorganic compounds in addition to protein are included in the non-rubber components. Substances revealing strain induced crystallization and having an accelerating effect, an antioxidant effect and a vulcanization-accelerating effect are included in these non-rubber components and play a role to bring out the excellent characteristics of natural rubber. On the other hand, non-rubber components exerting an adverse effect on a low hysteresis loss property and an abrasion resistance are present as well.
Accordingly, various problems are brought about when natural rubber latex is subjected to conventional deproteinizing treatment.
For example, if a conventional deproteinizing technique is used to completely remove protein, a large part of non-rubber components including protein is removed as well, and the preferred effects described above are damaged. On the other hand, if the non-rubber components are left remaining as they are, brought about is the deficiency that an adverse effect remains on the rubber physical properties such as a low hysteresis loss property and an abrasion resistance even if the preferred characteristics described above are maintained. Accordingly, non-rubber components contained in natural rubber are desired to exhibit mainly preferred characteristics when the natural rubber is used for a rubber composition and a rubber product using it.